To Assess Safety of Mitapivat and Provide Proof of Concept of the Efficacy of the Drug in Patients With RBC Membranopathies or CDAII.

This is a prospective exploratory phase 2 study designed to evaluate the safety and efficacy of mitapivat in RBC membranopathies and CDAII, a rare sub type of anemia. Nine patients from Princess Margaret who are diagnosed with CDAII will be enrolled to the study. Patients will be in the trial for 57 weeks of treatment and a safety follow-up visit after 30 days from last dose. First 8 weeks will be dose escalating period followed by 48 weeks of fixed dose period. 57th week will be dose tapering week, unless patient is deemed eligible for the fixed dose extension periods, in which case patient will continue on treatment for additional 24 months in total. Dose taper period will then occur on week 161 and safety follow-up will occur 4 weeks after last dose. Data collected from Princess Margaret will be incorporated to the main study conducted in EU for analysis. Overall, approximately 25 patients are expected to be enrolled: Approximately 16 patients at sites in the EU and approximately 9 patients in Canada.